1. Field of the Invention
This invention relates generally to a composition for the matrix body of rock bits and other cutting or drilling tools.
2. Background Art
Polycrystalline diamond compact (“PDC”) cutters are known in the art for use in earth-boring drill bits. Typically, bits using PDC cutters include an integral bit body which may be made of steel or fabricated from a hard matrix material such as tungsten carbide (WC). A plurality of PDC cutters is mounted along the exterior face of the bit body in extensions of the bit body called “blades.” Each PDC cutter has a portion which typically is brazed in a recess or pocket formed in the blade on the exterior face of the bit body.
The PDC cutters are positioned along the leading edges of the bit body blades so that as the bit body is rotated, the PDC cutters engage and drill the earth formation. In use, high forces may be exerted on the PDC cutters, particularly in the forward-to-rear direction. Additionally, the bit and the PDC cutters may be subjected to substantial abrasive forces. In some instances, impact, vibration, and erosive forces have caused drill bit failure due to loss of one or more cutters, or due to breakage of the blades.
While steel body bits may have toughness and ductility properties which make them resistant to cracking and failure due to impact forces generated during drilling, steel is more susceptible to erosive wear caused by high-velocity drilling fluids and formation fluids which carry abrasive particles, such as sand, rock cuttings, and the like. Generally, steel body PDC bits are coated with a more erosion-resistant material, such as tungsten carbide, to improve their erosion resistance. However, tungsten carbide and other erosion-resistant materials are relatively brittle. During use, a thin coating of the erosion-resistant material may crack, peel off or wear, exposing the softer steel body which is then rapidly eroded. This can lead to loss of PDC cutters as the area around the cutter is eroded away, causing the bit to fail.
Tungsten carbide or other hard metal matrix body bits have the advantage of higher wear and erosion resistance. The matrix bit generally is formed by packing a graphite mold with tungsten carbide powder and then infiltrating the powder with a molten copper-based alloy binder. For example, macrocrystalline tungsten carbide and cast tungsten carbide have been used to fabricate bit bodies. Macrocrystalline tungsten carbide is essentially stoichiometric WC which is, for the most part, in the form of single crystals. Some large crystals of macro-crystalline WC are bi-crystals. Cast tungsten carbide, on the other hand, generally is a eutectic two-phase carbide composed of WC and W2C. There can be a continuous range of compositions therebetween. Cast tungsten carbide typically is frozen from the molten state and comminuted to a desired particle size.
A third type of tungsten carbide used in hardfacing is cemented tungsten carbide, also known as sintered tungsten carbide. Sintered tungsten carbide comprises small particles of tungsten carbide (e.g., 1 to 15 microns) bonded together with cobalt. Sintered tungsten carbide is made by mixing organic wax, tungsten carbide and cobalt powders, pressing the mixed powders to form a green compact, and “sintering” the composite at temperatures near the melting point of cobalt. The resulting dense sintered carbide can then be crushed and comminuted to form particles of sintered tungsten carbide for use in hardfacing.
Sintered tungsten carbide is commercially available in two basic forms: crushed and pelletized. Crushed sintered tungsten carbide is produced by crushing sintered components into finer particles, the shape of which tends to be irregular and angular. Pelletized sintered tungsten carbide is generally rounded or spherical in shape. Spherical sintered tungsten carbide is typically manufactured by mixing tungsten carbide powder having a predetermined size (or within a selected size range) with a suitable quantity of cobalt or nickel, then formed into pellets (round globules). These pellets are sintered in a controlled atmosphere furnace to yield spherical sintered tungsten carbide. The particle size and quality of the spherical sintered tungsten carbide can be tailored by varying the initial particle size of tungsten carbide and cobalt, controlling the pellet size and adjusting the sintering time and temperature.
However, a bit body formed from the either cast or macrocrystalline tungsten carbide or other hard metal matrix materials may be brittle and may crack when subjected to impact and fatigue forces encountered during drilling. This can result in one or more blades breaking off the bit causing a catastrophic premature bit failure. Additionally, the braze joints between the matrix material and the PDC cutters may crack due to these same forces. The formation and propagation of cracks in the matrix body and/or at the braze joints may result in the loss of one or more PDC cutters. A lost cutter may abrade against the bit, causing further accelerated bit damage.
For the foregoing reasons, there is a need for a new matrix body composition for drill bits which has high strength and toughness, resulting in improved ability to retain blades and cutters, while maintaining other desired properties such as wear and erosion resistance.